In the field of mobile telecommunications, there is right now the opportunity to provide new mobile services which are beyond voice transmission. These messaging services are, however, presently limited to a transmission rate of 9,6 kbit/s.
Therefore, proposals for new technologies have been made how to increase the transmission bit rates. The solution for a packet switched data submission will most presumably be the General Packet Radio Service (GPRS) which is mainly intended for the interconnection to packet switched networks such as the Internet. However, for the connectivity to the existing circuit switched networks, the Integrated Services Digital Network (ISDN) and the Public Switched Telephone Network (PSTN), one of the mostly favored solution is the High Speed Circuit Switched Data (HSCSD) technology.
The reason therefor is that HSCSD enables the co-allocation of multiple full rate traffic channels into a HSCSD configuration. The aim of HSCSD is to provide a mixture of services with different air interface user rates by a single physical layer structure. The available capacity of a HSCSD configuration is several times the capacity of a full rate traffic channel, leading to a significant enhancement in the air interface data transfer rate.
Moreover, HSCSD uses a new channel coding scheme to enhance the bit rate of one time-slot from 9,6 kbit/s to 14,4 kbit/s. This increase is achieved by puncturing error correction bits of the 9,6 kbit/s channel coding. Hence, the transmission speed of only one time slot is 50% faster.
Thus, by combining these two features of HSCSD, a maximum HSCSD bit rate of 57,6 kbit/s uncompressed is available, which is comparable to one ISDN B-channel. Accordingly, when HSCSD is introduced, GSM data will be on par with existing PSTN modem services. For the technical specification of HSCSD, reference is made to document GSM 03.34 of the European Standards Telecommunications Institute.
However, the HSCSD technology is unfortunately associated with some problems. Since HSCSD uses several time slots for a single communications session, this multi-slot occupation is a heavy load for a single cell. Accordingly, when planning the provision of resources for the cells of a network, the traffic density of the network has to be taken into account. Since to date there is assumed a statistical distribution of the traffic density over time and space, this suffers from another significant drawback: If many subscribers use HSCSD simultaneously and thus occupying a rather huge amount of time slots, the actual traffic distribution can change considerably, probably leaving no time slot left for a later caller.
In addition, an introduction of new channel codings generally requires respective capabilities (e.g. hardware) to be implemented in a suitable location in a communication network.